The present invention generally relates to optical reflectometry, and more particularly, to a system and method for measuring a gap between two surfaces. With the advent of optical refletometry-based measuring devices capable of distances as small as 10 microns ({haeck over (s)}m), precise and accurate measurements critically small distances can be made. A nonlimiting example of an optical reflectometry-based measuring device is the optical thickness gauge (OTG) once sold by Hewlett-Packard (HP 86125A-KlX). The operation and functionality of such an OTG is disclosed in of U.S. Pat. No. 5,642,196, filed on Jun. 24, 1997, and entitled METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF A FILM USING LOW COHERENCE REFLECTOMETRY, which is entirely incorporated herein by reference. Other exemplary optical reflectometry-based measuring devices and their applications, incorporated herein by reference, are disclosed in U.S. Pat. No. 5,473,432, filed on Dec. 5, 1995, and entitled APPARATUS FOR MEASURING THE THICKNESS OF A MOVING FILM UTILIZING AN ADJUSTABLE NUMERICAL APERTURE LENS, U.S. Pat. No. 5,610,716, filed on Mar.11, 1997, and entitled METHOD AND APPARATUS FOR MEASURING FILM THICKNESS UTILIZING THE SLOPE OF THE PHASE OF THE FOURIER TRANSFORM OF AN AUTOCORRELATOR SIGNAL, US. Pat. No. 5,633,712, filed on May 27,1997, and entitled METHOD AND APPARATUS FOR DETERMINING THE THICKNESS AND INDEX OF REFRACTION OF A FILM USING LOW COHERENCE REFLECTOMETRY AND A REFERENCE SURFACES, U.S. Pat. No. 5,646,734, filed on Jul. 8, 1997, and entitled METHOD AND APPARATUS FOR INDEPENDENTLY MEASURING THE THICKNESS AND INDEX OF REFRACTION OF FILMS USING LOW COHERENCE REFLECTOMETRY, U.S. Pat. No. 5,731,876, filed on Mar. 24, 1998, and entitled METHOD AND APPARATUS FOR ON-LINE DETERMINATION OF THE THICKNESS OF A MULTILAYER FILM USING A PARTIALLY REFLECTING ROLLER AND LOW COHERENCE REFLECTOMETRY, and U.S. Pat. No. 5,850,287, filed on Dec. 15, 1998, and entitled ROLLER ASSEMBLY HAVING PRE-ALIGNED FOR ON-LINE THICKNESS MEASUREMENTS.
A conventional optical thickness gauge (OTG) is used to measured small distances between surfaces, such as a gap or separation between two materials. However, the OTG is limited in that there is some distance that is the smallest distance that the OTG can measure. That is, distances smaller than the smallest distance that the OTG can measure are less than the TG resolution capability, and therefore can not be determined. For example, one conventional type of OTG has a resolution of 10 microns (xcexcm). Distances less than 10 xcexcm can not be determined with a sufficient degree of accuracy and/or reliability.
FIG. 1 is a block diagram illustrating a conventional OTG 100 using a prior art method of measuring distances associated with a multi-layer film 102 and in communication with a personal computer (PC) 104. The OTG 100 has at least a low-coherence light source 106, an optical coupler 108, an autocorrelator 110 and a probe head 112. Low-coherence light 114 is generated by the low-coherence light source 106 and injected into waveguide 116. Waveguide 116 may be any suitable device, such as an optical fiber, that is configured to transfer the low-coherence light 114 to the optical coupler 108. The low coherence light 114 propagates through the optical coupler 108, through the waveguide 118 and into the probe head 112. Light is reflected back into the probe head 112, in a manner described below, through the waveguide 118, through the optical coupler 108, through the waveguide 120. The return light 122 is detected by the autocorrelator 110 so that distance measurements can be determined, as described below, by software (not shown) residing in PC 104.
For convenience of illustration, the waveguide 116 is illustrated as having a separation distance from the low-coherence light source 106. One skilled in the art will appreciate that the waveguide 116 would be typically coupled directly to the low-coherence light source 106 using well known coupling devices. Likewise, the waveguide 120 is illustrated as having some amount of separation from the autocorrelator 110. Waveguide 120 is typically coupled directly to the autocorrelator 110. For convenience of illustration, the waveguide 118 is illustrated as being directly coupled to the optical coupler 108 and probe head 112. Coupling devices used to couple the waveguides 116, 118 and 120 to devices are well known in the art and are not described in detail or illustrated herein. Furthermore, for convenience of illustration, the waveguides 116, 118 and 120 are illustrated as a rod-like material intended to represent a flexible optical fiber. However, any suitable waveguide device configured to transmit light between the low-coherence light source 106, the optical coupler 108, the autocorrelator 110 and the probe head 112, may be substituted for the waveguides 116, 118 and 120.
The optical autocorrelator 110 is configured to receive the return light 122. Detectors (not shown) residing in the autocorrelator 110 generates information such that the autocorrelator 110 generates correlation peaks that are shown on the graph 128. Separation between correlation peaks corresponds to distances between any two light reflecting surfaces.
Optical correlator 110 is coupled to the PC 104 via the connection 124. Information from the autocorrelator 110 is received by the PC 104 and processed by software (not shown) into correlation information. The PC 104 typically displays, on the display screen 126, the correlation results as a graph 128 having correlation peaks, described in greater detail below. That is, distances between correlation peaks correspond to the measurements taken by the OTG 100.
For convenience of illustration, the PC 104 is illustrated as a conventional laptop PC. However, any suitable PC or other processing device may be equally employed for the processing of information corresponding to the light signals received by the autocorrelator 110, and to prepare a meaningful output format that is interpreted by a user of the OTG 100 for the determination of distances. Furthermore, the display 126 may be any suitable device for indicating distance information resulting from measurements taken by the OTG 100. For example, but not limited to, the display 126 may be a conventional, stand-alone cathode ray tube (CRT). Or, a line printer, plotter, or other hard copy device may be configured to accept and indicate correlation information from the autocorrelator 110.
Light (not shown), entering the probe head 112 via the waveguide 118, first passes through a reference surface 130. Here, the reference surface 130 is illustrated as the bottom surface of a wedge-shaped plate 131. (For convenience of illustration, wedge-shaped plate 131 is shown from an edge-on viewpoint.) Reference surface 130 is configured to allow a portion of the received light to pass through the wedge-shaped plate 131 and onto the film 102. A portion of the received light (not shown) entering the wedge-shaped plate 131 is reflected from the reference surface 130, back through the probe head 112, through the waveguide 118, through the optical coupler 108 and then through the waveguide 120 to be received by the autocorrelator 110.
FIG. 2 is a simplified graph 200 illustrating the correlation peaks associated with the reflection of light from the reference surface 130 and the surfaces 132, 134, 136 and 138 of film 102 (FIG. 1) using the prior art method of measuring distances. For convenience of illustrating the autocorrelation information on the graph 200, the vertical axis corresponding to the magnitude of the correlation peaks is not numbered. One skilled in the art will realize that any appropriate vertical axis numbering system corresponding to the amplitude of the correlation peaks could have been employed, and that such a numbering system is not necessary to explain the nature of the correlation peaks. Similarly, the horizontal axis corresponding to distance has not been numbered on the graph 200. One skilled in the art will realize that any appropriate axis number system corresponding to distance could have been employed, and that such a numbering system is not necessary to explain the nature of the relationship between the correlation peaks illustrated in the graph 200. Thus, one embodiment of the software generating the graph 200 is configured to allow the user of the PC 104 to alter the horizontal and the vertical axis numbering systems so that the location of the correlation peaks of interest, and their relative separation corresponding to distance, can be meaningfully discerned and determined by the user of the PC 104 (FIG. 1).
Information received from the autocorrelator 110 is processed by the PC 104 (FIG. 1) such that the correlation peak 202 is plotted at the reference point (x=0) on the graph 200. Correlation peak 202 is a large peak, plotted at the zero or reference point on the x-axis of the graph 200, that corresponds to the correlation of each the reflected light portions with itself.
Returning to FIG. 1, the portion of light passing through the reference surface 130, referred to as the incident beam 140, passes through air for a suitable distance before striking the first surface 132 of film 102. When the incident beam 140 shines upon the surface 132, a portion of the incident beam 140 is reflected from the surface 132, as a reflected light beam 142, back up through the probe head 112, through the waveguide 118, through the optical coupler 108, through the waveguide 120, and then is received by the autocorrelator 110. The autocorrelator 110, based upon the time delay between the light reflected from the reference surface 130 and the reflected light beam 142, determines a correlation peak 204 (FIG. 2) as illustrated on the graph 200. Typically, the magnitude of the reflected light beam 142 is relatively small. Thus, the correlation peak 204 is significantly less in magnitude than the correlation peak 202, as illustrated in the graph 200. The user of the PC 104 viewing the graph 200 interprets the relative separation between the correlation peaks 202 and 204 as corresponding to a distance 144 between the referenced surface 130 and the surface 132 of the film 102.
For convenience of illustration, the incident beam 140 and the reflected light beams 142, 154, 158 and 162 reflected from surfaces 132, 134, 136 and 138, respectively, are shown at slight angles. However, one skilled in the art will appreciate that the incident beam 140 and the light beams 142, 154, 158 and 162 are all orthogonal to the reference surface 130 and the surfaces 132, 134, 136 and 138. Furthermore, for convenience of illustration, because the distance 144 is typically much greater than the distances of interest associated with the film 102, only a portion of the distance between the correlation peaks 202 and 204 is illustrated. Thus, a portion of the horizontal axis and a portion of the distance between the correlation peaks 202 and 204 is omitted from the graph 200, as indicated by the break line 206.
One skilled in the art will appreciate that the separation between the correlation peaks 202 and 204 is function of a variety of well known physical factors. Light travels at a finite speed. The speed of the light is affected by the medium through which the light is traveling. Thus, one skilled in the art will readily appreciate that two significant factors in determining the time delay of the various portions of light detected by the autocorrelator 110 are the total distance traveled by the light, and the properties of the various medium through which the light travels. For example, the reflected light beam 142 travels from the reference surface 130 to the surface 132, and then returns back to the reference surface 130. Therefore, because the reflected light beam 142 travels farther than the light reflecting from the reference surface 130, and because the light beam 142 travels through air, the light beam 142 requires more time to reach the autocorrelator 110 than the time required by the light reflecting from the reference surface 130. The physical properties associated with the mediums through which the light travels is defined by the well known refractive index (n) of the material. Thus, software analyzing the relative separation between the correlation peak 202 and the correlation peak 204 accurately calculates the distance 144 and provides that information to the user of the PC 104. This information is communicated by appropriately labeling the horizontal axis of FIG. 2, and/or providing a numerical figure to the user. Such a process of determining distances with an OTG 100 (FIG. 1) is well known in the art and is not described in further detail herein.
FIG. 1 illustrates the OTG 100 measuring distances associated with film 102. For convenience of illustration, the film 102 has three layers; a top layer 146, a middle layer 148 and a bottom layer 150. The layers 146, 148 and 150 are made from different materials bonded together to create a single layer of film 102. Typically, the film 102 is a long, continuous roll or sheet of flexible material. However, for convenience, only a portion of the roll or sheet of the film 102 is shown in FIG. 1, as illustrated by the cut-away lines 152. Furthermore, the layers 146, 148 and 150 must be sufficiently transparent so the incident beam 140 travels through, and light reflected back through the layers 146, 148 and 150.
Each layer 146, 148 and 150 have different refractive index (n). Surface 132 corresponds to the transition between air and the film 102, and thus corresponds to a change in the refractive index of air to the refractive index of the top layer 146. Similarly, surface 134 corresponds to the transition between the material of top layer 146 and the material of middle layer 148. Surface 136 corresponds to the transition between the middle layer 148 and the bottom layer 150. Surface 138 corresponds to the bottom surface of film 102, and also corresponds to a transition between the bottom layer 150 and the material that the film 102 is residing in, such as air. Each of these surfaces are also characterized by a change in refractive index.
When the incident beam 140 is incident on the surface 134, a portion of the incident beam 140 passes through the surface and a portion of the incident beam 140 is reflected back up to the probe head 112 because of the difference in the refractive index n of the layers 146 and 148. The amount of reflected light corresponds, in part, to the degree of difference between the refractive index n. Thus, when the incident beam 140 passes through the top layer 146 into the middle layer 148, the reflected light beam 154 is reflected from the surface 134 back up through the top layer 146 and into the probe head 112. The reflected light beam 154 is eventually detected by the autocorrelator 110 in the manner described above. Because of the time delay between the reflected light beam 154 from the surface 134 with respect to the light reflected from the reference surface 130, a correlation peak 208 (FIG. 2) will be determined. Furthermore, since the time delay between the reflected light beam 154 from the surface 134, with respect to the reflected light beam 142 from the surface 132, is equal to the time required for the light to travel through the layer 146 only, the separation between correlation peak 204 and correlation peak 208 is proportional to the distance 156 and the index of refraction of the layer 146.
Similarly, a portion of the incident beam 140 incident on the surface 136 corresponding to the material transition between the middle layer 148 and the bottom layer 150, is reflected back up to the probe head 112 as reflected light beam 158. Because of the time delay associated with the reflected light beam 158 with respect to the light reflected from the reference surface 130, a correlation peak 210 (FIG. 2) is determined. Furthermore, since the time delay between the reflected light beam 158 from the surface 136, with respect to the reflected light beam 154 from the surface 134, is equal to the time required for light to travel through the layer 148 only, the separation between the correlation peak 208 and the correlation peak 210 is proportional to the distance 160 in the index of refraction of the layer 148.
Likewise, a portion of the incident beam 140 will be reflected from the surface 138 back up to the probe head 112 as a reflected light beam 162. Because of the time delay associated with the reflected light beam 162 with respect to the light reflected from the reference surface 130, a correlation peak 212 (FIG. 2) is determined. Furthermore, since the time delay between the reflected light beam 162 from the surface 138, with respect to the reflected light beam 158 from the surface 136, is equal to the time required for light to travel through layer 150 only, the separation between the correlation peak 210 and the correlation peak 212 is proportional to the distance 164 and the index of refraction of the layer 150. In some applications, the bottom surface 138 of the film 102 is coated with a highly reflective surface to cause a large portion of the incident beam 140, or all of the remaining incident beam 140, is reflected up to the probe head 112 as the reflected light beam 162. Thus, the correlation peak 212 is illustrated as having a relatively greater magnitude than the correlation peaks 204, 208 and 210 (FIG. 2).
The autocorrelator 110 (FIG. 1) generates a correlation peak for all pairs of reflections from any two surfaces. However, for convenience of illustrating the graph 200 (FIG. 2), not all correlation peaks are illustrated. When spatial separation between the film surfaces 132, 134, 136 and 138 (FIG. 1) are sufficient, correlation peaks generated by the correlation of the reference surface 130 with each of the film surfaces 132, 134, 136 and 138 are used to make measurements of the thickness of the film layers 146, 148 and 150 (FIG. 1). Alternatively, the top surface 132 may be used instead of reference surface 130 to determine correlation peaks.
One skilled in the art will appreciate that many correlation peaks will be displayed on the graph 200, and that one skilled in the art will employ experience in using the OTG 100 (FIG. 1) to determine which correlation peaks are relevant to the particular measurements of interest. Thus, for convenience of illustration, the correlation peaks illustrated on the graph 200 are limited to correlation peaks that are convenient in explaining the operation and functionality of the OTG 100.
Summarizing, the OTG 100 shines a low-coherence incident beam 140 onto the film 102 so that portions of the incident beam 140 are reflected back to the OTG 100 (reflected light beams 142, 154, 158 and 162) and detected by the autocorrelator 110. Software analyzes the time delays associated with the reflected light beams 142, 154, 158 and 162, with respect to the light reflected from reference surface 130, to determine the distances 144, 156, 160 and 164, respectively. The ability to resolve the minimum peak separation is determined by the coherence-length of the light source. Thus, a lower-coherence length light source gives a higher resolution. One commercially available OTG 100 is capable of discerning distances as small as 10 xcexcm.
However, the above-described commercially available OTG 100 is not capable of measuring with any degree of reliability and accuracy of distances smaller than 10 xcexcm. Even as technologies advance such that the resolution of more advanced OTGs provide for measuring distances smaller than 10 xcexcm, there will always be some minimum distance that an OTG is able to measure within an acceptable degree of reliability and accuracy. Distances less that this minimum distance can not be measured with an acceptable degree of reliability and accuracy. Thus, a heretofore unaddressed need exists in the industry for providing a system and method of accurately and reliably measuring distances that are smaller than the minimum distance that an OTG can reliably and accurately measure.
The present invention reliably and accurately measures a gap between two materials when the depth of the gap is less than the smallest distance that the measuring device, such as an optical thickness gauge (OTG), is able to reliably and accurately measure. For example, if an OTG is capable of measuring distances as small as 10 microns (xcexcm), the invention allows accurate and reliable measurement of a gap having a distance that is smaller than 10 xcexcm. The invention is practiced by forming a suitable recess in at least one of the materials. Examples of such a recess include a slot, groove, channel, hole or other suitable deformation. The depth of the recess is precisely known. Thus, the sum of the distance of the gap and the depth of the recess is at least equal to the smallest distance that the OTG can measure with an acceptable degree of reliability and accuracy. The recess may be formed in either material. In an alternative embodiment, the recess is formed in both materials.
The recess is positioned over the materials and under the probe head of the OTG to form a measurable region or cavity. The depth of the cavity is precisely measured. Since the distance of the recess is precisely known, and the depth of the cavity is measurable, the depth of the gap is easily determined by subtracting the known distance of the recess from the measured depth of the cavity. Thus, the inclusion of the recess in at least one of the materials enables the OTG to accurately and reliably determine the depth of the gap. Hereinafter, the term xe2x80x9cslotxe2x80x9d is used interchangeably with the term xe2x80x9crecessxe2x80x9d for convenience.
In another embodiment, the depth of the slot is not precisely known when the slot is formed in the material. However, the depth of the slot is at least equal to the distance that the OTG can reliably and accurately measure. Thus, the depth of the slot is determinable by measurement.
The present invention can also be viewed as providing a method for measuring distance between two materials. The method includes the steps of measuring a distance between a slot surface formed by a slot in a first material and a surface on a second material (the first material having a precisely known distance between the slot surface and the surface of the first material); and subtracting from the measured distance the precisely known distance to determine the distance between the first material and the second material.
Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following detailed description, when read in conjunction with the accompanying drawings. It is intended that all such features and advantages be included herein within the scope of the present invention and protected by the claims.